The present disclosure relates to the field of data processing, and more specifically to preserving pure gray colors during color conversion.
Digital devices that create (e.g., scanners and digital cameras), display (e.g. CRT and LCD monitors), or print (e.g. ink-jet and laser printers) colors typically define color data using color spaces. Generally, a color space is a combination of a color model and a gamut. A color model defines each color within the model using primary color components, such as, in the case of a Red, Green, Blue (RGB) color model, the levels of red, green, and blue light components needed to create each color, or in the case of a Cyan, Magenta, Yellow, and Key (CMYK) color model, the levels of cyan, magenta, yellow, and black ink needed to create each color. Levels of each component the color models typically range from 0 to 100 percent of full intensity. By varying the levels or intensities of the components, each color in the color model may be created. However, as a practical matter a device is often limited in its ability to create pure red, green, or blue light, which limits its range of colors or color gamut. A gamut is simply the range of colors that may be displayed on, rendered by, or captured by a particular device.
The differences in device gamuts and primary color definitions lead to differences in color spaces between devices. For example, two devices that use RGB may show different colors when each displays its most intense red. The most intense red on a first device may have an intensity of 1 for the R component and 0 for the G and B components. However, the color that looks the same as the most intense red of the first device may have a red intensity of 0.85 on a second device. Moreover, the G and B component intensities may even be 0.05 on the second device. In other words, the same perceived “red” color has different RGB component values depending on the device, on the first device it may be (1, 0, 0) and on the second device that same “red” may be (0.85, 0.05, 0.05). This means that an image file containing only RGB values, if displayed directly by both devices, would appear differently on the two devices.
To solve this problem of the same component values appearing differently on different devices, color spaces can be defined in relation to device-independent color spaces, which define colors in more absolute terms. Some examples of device-independent color spaces include the CIE XYZ and CIE L*a*b* color spaces. The relationship of a device's native color space with a device-independent color space typically is described by some combination of formulas, transfer functions, matrices, and look up tables. This relationship may be stored in an International Color Consortium (ICC) or other color profile for the device. The device-independent color space may be used as an intermediate when converting from one device-dependent color space to another.
The conversion from one color space to another may be done through a series of conventional processing steps. Some processing steps may be more computationally intensive than others. Some processing steps may require interpolation. Generally, there is a tradeoff between the number of steps, the complexity of each step, speed, and accuracy. In some applications, speed is of the essence and accuracy is sacrificed by reducing the number of steps and/or the complexity of the individual steps. Often to increase speed, a look up table (LUT) is used either alone or with another simple processing step. A LUT maps points in one color space to corresponding points in another color space. For example, a color in a first RGB color space may have the color component values of (0, 45, 82) which, when converted to a second RGB color space, the color may have the color component values (5, 51, 76). This is because the color profile for each color space defines pure R, G, and B components differently. A LUT may be constructed by transforming a regularly spaced grid of colors in a first color space to a second color space using the most accurate processing steps, such as using the color profiles for example. Each grid point and its corresponding transform point in the second color space may be stored in the LUT. Converting colors that do not correspond to the grid points would involve interpolation, therefore, the more grid points the more accurate the conversion. However, increasing the number of the grid points complicates the LUT and may result in an increase in processing time.
Gray colors are often specified using the K channel of a CMYK color space. When colors are converted from one CMYK color space to another CMYK color space, such as in the case of going from editing to printing, colors having only a K component value (e.g., (0,0,0,50)) may be converted to a four channel CMYK color (e.g., (21,22,33,34)), which may result in an undesirable image or wasted ink.